1. Field of the Invention
This invention relates to a device and method for mixing solid and liquid substances by suitable mixing action, and more particularly to a mixing device for mixing various kinds of pulverulent materials such as cement with a liquid such as water to obtain a suitable intimate mixture in which the pulverulent materials are uniformly dispersed in the mixture by rotating paired mixing blades so as to cause collisions of the particles of the pulverulent materials.
2. Description of the Prior Art
The quality of a mixture of various pulverulent materials and a liquid such as water depends on whether or not the particles of the pulverulent materials are uniformly dispersed in the resultantly obtained mixture and mixed completely with the liquid without being left in the form of immiscible lumps with the liquid. For example in the case of producing ready-mixed concrete, it is frequently difficult to produce concrete having satisfactory strength by mixing cement and aggregate with water to obtain cement paste in which the cement and aggregate are consistently mixed with the water. Therefore, there is now being widely adopted a step mixing method for producing concrete, in which cement and water are first mixed to make cement paste, and thereafter, aggregate is added to and mixed with the cement paste thus made.
One mixing device for carrying out the step mixing method for making such cement paste has been proposed by Japanese Patent Publication SHO 61-7928(B). In this conventional mixing device, raw materials such as water and cement are admitted one into either end portion of a mixing drum and forcibly introduced into a pressure chamber located at the center of the mixing drum by driving a screw to mingle the raw materials and obtain cement paste, and then, the cement paste thus obtained is fed out at a high rate of speed through a contracted path to impart a shearing force thereto.
The pulverulent particles of raw materials to be mixed tend to remain in a resultantly obtained mixture such as cement paste in the form of immiscible lumps in water, which lumps accommodate air between the pulverulent particles in such a state that the water brings about surface tension in the mixture. Therefore, fine air bubbles in the lumps of the pulverulent materials are held in the mixture by the liquid film crosslinking effect. Although there is a case that the pulverulent particles are electrostatically joined with one another, the coherence of the pulverulent particles joined electrostatically is weaker than the coherence caused by the liquid film crosslinking effect. However, in any case, the lumps of the pulverulent materials which are first brought about in the mixture cannot easily be broken merely by driving the screw used in the aforenoted conventional mixing device, because the fine air bubbles in the lumps of the pulverulent materials possibly serve as a cushion.
Furthermore, in the conventional mixing device noted above, heavy particles of the pulverulent materials in the mixture precipitate to the bottom of the mixing drum because of the difference in specific gravity of the particles. Therefore, the shearing force imparted to the cement paste occurs only in the contracted path in the mixing drum, but has no effect on the pulverulent particles precipitated to the bottom of the mixing drum.
As described above, the lumps of the pulverulent materials can not be broken effectively by the prior art apparatus including the conventional mixing device as noted above.
The inventors of the present invention have carried on various studies of an energy introducing method for effectively breaking and uniformly dispersing lumps of pulverulent particles in the mixture.
From the results of their studies, it was found that the finer the pulverulent particle to be mixed with a liquid having surface tension is, the larger the cohesion thereof becomes. To be more specific, the pulverulent particles such as cement generally have a diameter of the order of about 10 .mu.m to several ten .mu.m, and such fine particles tend to gather and agglomerate to form a lump in a liquid. Therefore, by the conventional mixing device which acts to mechanically mix the pulverulent materials with a rotating blade or one set of blades in a mixer container, a kinetic energy sufficient to break down the lumps of the pulverulent particles cannot be produced. That is to say, it is required to impart the energy directly to the lumps of the pulverulent particles in order to consistently mix the pulverulent particles with water. Further, the study made by the inventors reveals that the pulverulent particles can be dispersed uniformly in a liquid by causing collision of the particles by utilization of the difference in inertial mass between the pulverulent particles, and therefore, it is desirable to cause the pulverulent particles to collide head-on with one another.